1. Field of the Invention
The present invention is related to fabrics having a fiber structure to which is applied low glass transition temperature viscous polymer adhesives.
2. Description of Related Art
Current soft body armor systems made from woven fabrics require high weight density per unit area, partly in order to achieve less than 44 mm back face deformation (BFD) as required by NIJ standard 0101.04 Rev. A. BFD is an indicator of blunt trauma, the lower the BFD, the better the protection from blunt trauma. Although many soft body armor constructions can adequately stop ballistic projectiles, the shock associated with blunt trauma can still cause substantial injury or death. Consequently, high-end lightweight vests typically use hybrids of woven fabrics with substantial amounts of nonwoven laminated structures, such as Honeywell's Goldflex® or Spectrashield®.
U.S. Pat. No. 5,776,839 discloses the application of dilatant dry powders, with a typical composition consisting of carbon black, fumed silica (nano-silica), and a small amount of adhesive “glue” to ballistic fibers and fabrics.
U.S. Pat. No. 5,229,199 considered rigid composites of woven aramid fabrics coated with an adhesion modifier and imbedded in a matrix resin. The reduced friction and weakened interfaces led to improved ballistic performance. If friction is too high in a fabric, or if the matrix is too stiff, ballistic resistance is severely compromised.
Lee, Y. S. et al. (N.J. Advanced Body Armor Utilizing Shear Thickening Fluids, 23rd Army Science Conference, 2002) consider shear-thickening suspensions of particles in conjunction with ballistic fibers.
U.S. Pat. No. 4,678,702 discloses a protective laminate formed by bonding layers of Kevlar® fabric together by layers of Surlyn® under heat and pressure to cause the Surlyn® to flow into and encapsulate the yarn of the Kevlar® fabric.
Fabrics impregnated with solid adhesives, such as polyethylene film, in monolithic systems could lead to a significant loss of penetration resistance due to a combination of high stiffness, the processing conditions needed for impregnation and to the absence of some of the ballistically significant material, such as Kevlar®, a polyaramid available from E.I. du Pont de Nemours and Co, Wilmington, Del. (DuPont).
Woven fabric based soft body armors typically exhibit large BFD, thereby requiring higher basis weight for compliance with NIJ standard 0101.04 Rev. A. For example, current vests made of 100% woven Kevlar® can weigh more than 1.0 psf to achieve Level II protection under the NIJ standard. Some film-impregnated fabrics, (such as those with polyethylene film) are typically used in conjunction with untreated fabric layers, with the impregnated layers placed nearer the body to control BFD. Such hybrid systems are needed in order to compensate for the weight gain associated with film lamination and to minimize overall stiffness of the assembly. Nevertheless, such solutions often significantly compromise ballistic resistance.